ASSISTANT PROFESSOR DEPARTMENT OF CHEMISTRY PADMAVANI ARTS AND SCIENCE COLLEGE FOR WOMEN SALEM11 Stereochemistry The study of the threedimensional structure of molecules Structural constitutional isomers ID: 1033931
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1. Organic Chemistry-IC. SAKTHIVELASSISTANT PROFESSOR DEPARTMENT OF CHEMISTRYPADMAVANI ARTS AND SCIENCE COLLEGE FOR WOMEN, SALEM-11
2. Stereochemistry:The study of the three-dimensional structure of moleculesStructural (constitutional) isomers:same molecular formula but different bonding sequenceStereoisomers:same molecular formula, same bonding sequence, different spatial orientationTypes of StereoisomersTwo types of stereoisomers:enantiomerstwo compounds that are nonsuperimposable mirror images of each otherdiastereomersTwo stereoisomers that are not mirror images of each otherGeometric isomers (cis-trans isomers) are one type of diastereomer.UNIT-1Stereochemistry
3. ChiralEnantiomers are chiral:Chiral:Not superimposable on its mirror imageMany natural and man-made objects are chiral:handsscissorsscrews (left-handed vs. right-handed threads)Right hand threads slope up to the right.Some molecules are chiral:Asymmetric (chiral) carbon
4. Chiral vs. AchiralTo determine if a compound is chiral:0 asymmetric carbons: Usually achiral1 asymmetric carbon: Always chiral2 asymmetric carbons: Chiral or achiralDoes the compound have an internal plane of symmetry?Yes: achiralNo:If mirror image is non-superimposable, then it’s chiral.If mirror image is superimposable, then it’s achiral.Example: Identify the following molecules as chiral or achiral.trans-1,3-dibromocyclohexaneethylcyclohexane
5. (R) And (S) NomenclatureStereoisomers are different compounds and often have different properties.Each stereoisomer must have a unique name.The Cahn-Ingold-Prelog convention is used to identify the configuration of each asymmetric carbon atom present in a stereoisomer.(R) and (S) configurationThe two enantiomers of alanine are:Natural alanineUnnatural alanine(S)-alanine(R)-alanineExample priorities:I > Br > Cl > S > F > O > N > 13C > 12C > 3H > 2H > 1H
6. Chirality of Conformational IsomersChirality of Conformational IsomersThe two chair conformations of cis-1,2-dibromocyclohexane are nonsuperimposable, but the interconversion is fast and the molecules are in equilibrium. Any sample would be racemic and, as such, optically inactive.
7. Fischer Mirror ImagesFisher projections are easy to draw and make it easier to find enantiomers and internal mirror planes when the molecule has two or more chiral centers.CH3HClClHCH3Chapter 57
8. Fischer (R) and (S)Lowest priority (usually H) comes forward, so assignment rules are backward!Clockwise 1-2-3 is (S) and counterclockwise 1-2-3 is (R).Example:(S)(S) CH3HClClHCH3Chapter 58
9. Diastereomers: Cis-trans Isomerism on RingsCis-trans isomers are not mirror images, so these are diastereomers. Chapter 59
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